There is an air flowmeter as an example of the heating resistor type flow-measuring device. This air flowmeter comprises: a heating resistor (heater); a current control circuit (feedback control using a bridge circuit) for maintaining the temperature of this heating resistor to a constant temperature; a temperature sensor disposed near the heating resistor, and a thermoresistance whose resistance value changes in response to the ambient temperature (a flow sensor described in the patent document 1, and a heater drive circuit described in the patent document 2).
[Patent document 1] JP-A No. 310762/2002
[Patent document 2] JP-A No. 314645/2000
Generally, in the heating resistor type flow-measuring device, heating temperature of the heating resistor needs to be initially adjusted.
In a technology disclosed in the patent documents 1, in order to adjust the temperature of a heating resistor, resistance values of bridge resistors that constitute the bridge circuit together with the heating resistor is adjusted, and in order to adjust the resistance value of the bridge circuit, the technology adopts a method of changing connection of resistor pattern by wire bonding.
If the technology can adjust the heating temperature when the ambient temperature rises to reach a temperature causing large aging (for example, 350° C.), in addition to its capability of initial adjustment, it will be convenient.
However, with the technology disclosed in the patent document 1, the initial adjustment of the heating resistor is cumbersome and requires a lot of time, and in addition, after the initial adjustment of the heating resistor, it is difficult to alter the heating temperature properly according to a change in the ambient temperature.
On the other hand, in the heater drive circuit described in the patent document 2, in order to adjust the temperature of the heating resistor (heater), one of bridge resistors that constitute a bridge circuit together with the heating resistor is chosen to be a variable resistor and the resistance value of this variable resistor is adjusted.
When adjusting the resistance value of this variable resistor based on digital information, it is conceivable to apply a circuit configuration as shown in FIG. 8.
That is, the variable resistor circuit shown in FIG. 8 is configured in such a way that resistors 53, 54, 55, 56, 57 each of which is connected to MOS transistors 48, 49, 50, 51, 52, respectively to form a plurality of serially-connected elements of a resistor and a MOS transistor, which are then connected to one another in parallel between the terminal A and terminal B.
Then, by turning these MOS transistors 48, 49, 50, 51, 52 ON or OFF based on digital information, a combined resistance between terminals A and B can be changed.
However, in order to adjust the temperature of the heating resistor with high precision, it is necessary to alter the combined resistance of the circuit whose configuration is shown in FIG. 8 by the unit of several ohms. For this reason, it is necessary to set ON resistance values of the MOS transistors 48, 49, 50, 51, and 52 to several ohms or less. However, for this purpose, the sizes of the MOS transistors 48, 49, 50, 51, 52 need to be increased, and consequently the heating resistor type flow-measuring device becomes large and heavy in weight. In the case where this variable resistor circuit is implemented in an integrated circuit, extremely large chip size is required.
Because of this, it is difficult to attain small size and lightweight and to adjust the heating temperature of the heating resistor according to the ambient temperature as well as making the initial adjustment.